Biochar Hybrid Materials: Photocatalytic

Research Article pubs.acs.org/journal/ascecg

Dual Functionality of TiO2/Biochar Hybrid Materials: Photocatalytic Phenol Degradation in the Liquid Phase and Selective Oxidation of Methanol in the Gas Phase Paweł Lisowski,*,† Juan Carlos Colmenares,*,† Ondřej Mašek,‡ Wojciech Lisowski,† Dmytro Lisovytskiy,† Agnieszka Kamińska,† and Dariusz Łomot† †

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warsaw 01-224, Poland UK Biochar Research Centre (UKBRC), School of Geosciences, University of Edinburgh, Edinburgh EH8 9YL, United Kingdom

‡

S Supporting Information *

ABSTRACT: A series of new inorganic−organic hybrid materials based on TiO2 and new biochar-based supports (biochar obtained by pyrolysis of miscanthus straw pellets (MSP) and soft wood pellets (SWP) at 550 and 700 °C) were successfully prepared using ultrasound-assisted methodology. The resulting composites were characterized by a wide range physicochemical techniques and investigated in water and gasphase photocatalytic test reactions. Our best composite (TiO2/SWP700) achieved phenol degradation of 64.1% (under UV light) and 33.6% (under visible light). In addition, it also showed an extraordinarily high activity (∼90%) in selective oxidation of methanol to methyl formate in flow gas phase, high selectivity to methyl formate (∼80%), and high yield of methyl formate (∼88%) after 240 min of illumination. It can be noticed that when TiO2 is supported on biochar it presented a superior photocatalytic ability and could be recycled at least 5 times in both photocatalytic runs tests with reproducible high photocatalytic efficiency. KEYWORDS: Biochar, TiO2, Synthesis of composite photocatalysts, Sonication for composite preparation, Phenol degradation, Methanol oxidation

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INTRODUCTION Photocatalytic technology offering low cost, high reactivity, and easy recovery is of great relevance and significance for energy efficiency and environmental remediation. In recent years, TiO2 has been extensively studied as an eminent material for its wide applications in photocatalysis due to its outstanding photocatalytic activity, low cost, extraordinary stability with minimum photocorrosion, and so on.1,2 Combining TiO2 with nontoxic and abundant carbon materials has been employed in the study of novel possibilities for the design of photocatalysts which can show stable performance for heterogeneous photocatalysis.1−4 Recently, the great application potential of biochar from pyrolysis of biomass has received increasing attention, and there is a growing awareness of biochar’s ability to provide a versatile and efficient platform for the synthesis of functionalized carbon materials.5,6 Biochar can be defined as a porous solid-rich byproduct of thermal decomposition of wastes for nonfuel uses.7,8 An important advantage of biochar over more conventional activated carbon is the presence of surface functional groups (e.g., phenolic hydroxyl, carbonyl, and carboxyl groups) on the surface of biochar.7−10 Worth mentioning is the work by Matos10 on the preparation of biochar-based functional materials and investigation of a wide © 2017 American Chemical Society

spectrum of harmful compounds in water and air under visible light irradiation. Purification of water and air may be problematic with global relevance, and heterogeneous photocatalysis has attracted considerable attention as a potential technique for pollution remediation. In this study we selected methanol and phenol as target compounds for photocatalytic treatment. Methanol was specified by The United States Environmental Protection Agency (U.S. EPA) as an important pollutant which may have significant negative impacts on natural environments.11,12 Phenol is on the U.S. EPA’s list of priority pollutants and has potential for causing different effects on human health.13,14 Recent years have witnessed a rapid preparation of highly potent photoactive materials using ultrasound-based procedures which offer a facile, versatile synthetic tool and have great potential for the future of photocatalyst preparation.15,16 Ultrasound-based procedures ensure extremely highly localized pressures and temperatures in liquid-phase reactions owing to the cavitation effect which enhances the chemical reactivity.15,16 Received: April 22, 2017 Revised: May 24, 2017 Published: May 30, 2017 6274

DOI: 10.1021/acssuschemeng.7b01251 ACS Sustainable Chem. Eng. 2017, 5, 6274−6287

Research Article

ACS Sustainable Chemistry & Engineering

handling screw conveyor, a collection vessel, and an afterburner chamber. The unit can operate at temperatures of up to 850 °C and achieve mean residence times of solids from a few minutes to over 40 min. The chars used in this work were produced from softwood pellets (SWP) and miscanthus straw pellets (MSP) at peak pyrolysis temperatures of 550 and 700 °C and a residence time at a peak temperature of around 5 min. Table S1 (see Supporting Information) contains basic characteristics of the four standard biochars used. Preparation of Hybrid TiO2-Based Biochar Materials. Prior to synthesis, biochar materials were washed thoroughly three times in boiling Milli-Q water and subsequently oven drying (110 °C). After optimizing the TiO2 loading (15, 25, and 35 wt %) it can be stated that 25 wt % loading of TiO2 on biochar gave optimal performance, and therefore, this loading level was used in the preparation of all four biochar-supported TiO2 catalysts in this study, as shown in Figure 2. TiO2-based biochar materials were prepared following ultrasoundbased procedures (surfactant-free synthesis) as previously shown in ref 17. In brief, 500 mg of biochar was placed into a solution of titanium(IV) isopropoxide (TTIP) (0.47 mL; 1.5 mmol) in 2propanol (total volume ratio of TTIP:2-propanol was 1:40), and the whole mixture was sonicated for 1 h (ultrasonic bath, frequency 35 kHz, 560 W, Sonorex Digitec-RC, Bandelin). The solvent was removed using a rotary vacuum evaporator assisted by sonication. The powder material was further dried for 3 h at 110 °C and subsequently after drying calcined in a furnace at 400 °C at a heating rate of 3 °C min−1 for 5 h in an oxygen-deficient atmosphere (static air). The final samples were denoted as TiO2/MSP550, TiO2/ MSP700, TiO2/SWP550, and TiO2/SWP700, where the second part indicates the type of biochar support used. For comparative purposes, TiO2/NORIT (NORIT, commercially available activated carbon) was prepared by ultrasound-assisted wet impregnation and TiO2/SWP700 was prepared without ultrasound.

On the basis of the above considerations, a series of new inorganic−organic hybrid materials based on TiO2 and new biochar-based supports was successfully prepared using the ultrasound-assisted wet impregnation method.17 The photocatalytic activity and selectivity of prepared composites were performed using our reaction system for selective oxidation of methanol in the gas phase and phenol photodegradation in the liquid phase. To the best of our knowledge, this is the first complete work on the photocatalytic oxidation of methanol in the gas phase to form methyl formate and photocatalytic degradation of phenol by using biochar’s modified on its surface with TiO 2 prepared by the ultrasound-promoted wet impregnation methodology. The use of such nonconventional procedures for materials synthesis is very attractive from the perspective of green chemistry as it reduces costs and energy consumption.15,16

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EXPERIMENTAL SECTION

Preparation of Biochar by Pilot-Scale Continuous Pyrolysis. The biochars used as TiO2 support in this study belong to the socalled Edinburgh Standard Biochar set (www.biochar.ac.uk/standard) and were produced using the UKBRC Stage III Pyrolysis Unit (rotary kiln pyrolyser) at the University of Edinburgh, shown in Figure 1. The

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RESULTS AND DISCUSSION Textural properties of the prepared materials are summarized in Table S2 (see Supporting Information), including the specific surface area and pore analysis. It was noted that the BET specific surface area of the TiO2/biochar materials increased compared to untreated biochar. Comparing the specific surface area of pure biochar materials like SWP550 and SWP700 (107 and 254 m2 g−1, respectively), it should be noted that the most significant increase in the BET area is for TiO2/SWP550 and TiO2/SWP700 (400 and 399 m2 g−1, respectively) in comparison with other photocatalysts with the same TiO2 content. As displayed in Figure S1 (see Supporting

Figure 1. Rotary kiln pilot-scale pyrolysis unit (Stage III) at the UKBRC, University of Edinburgh (source Ansac Ltd.). rotary kiln pyrolysis unit consists of a biomass feeder, sealed rotating drum (i.d. = 0.244 m, heated length 2.8 m) heated by a set of electric heaters arranged in three heater banks of 16.67 kW each, a char

Figure 2. Synthesis procedure for hybrid TiO2/biochar materials using ultrasound-based procedures.17 6275


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ACS Sustainable Chemistry & Engineering Information), the nitrogen adsorption−desorption isotherms of TiO2/MSP550, TiO2/MSP700, and TiO2/SWP550 indicate the presence of slit-shaped pores18,19 associated with plate-like particles, which is characteristic for microporous carbon materials. According to IUPAC classification, TiO2/SWP700 exhibits the four types of H4-hysteresis loops, pointing out that the prepared composite has mesoporous structure20 with platelike particles which usually give rise to narrow slit-shaped pores. Toward a better understanding of the influence of ultrasound, TiO2/SWP700 without ultrasound was prepared. Results indicated a 65% of BET surface area increase, together with a rise in external BET in relation to TiO2/SWP700 prepared by the ultrasound-assisted wet impregnation method compered to TiO2/SWP700 prepared by the wet impregnation method (without ultrasound). It is worthy of note that the results obtained by the ultrasound-assisted wet impregnation method showed a significant effect of sonication on the textural properties. On the basis of these findings, it is likely that the larger external surface area, e.g., in the case of TiO2/SWP700, would provide more opportunities for interaction of reactants with active sites of the photocatalyst and thus lead to greater activities. Additionally, a higher BET of prepared materials may be ascribed to the effect of ultrasonic irradiation because it can produce cracks and increase the mesoporous structure of TiO2/ SWP700. In addition, it can also result in formation of more photocatalytic surface-active centers and sites and thus is favorable to the improvement of the photocatalytic performance. This is owing to both a larger number of photocatalytic surface-active centers and adsorption sites for compounds and the increase of the ease reactants’ transport through the network of interconnected pores. The plate-like particle structures are considered to be beneficial in utilizing more light and effective separation of photogenerated charge.18,19 The morphology and microstructure of prepared TiO2/ biochar were investigated with SEM as revealed in Figure 3.

Figure 4. X-ray diffraction (XRD) patterns for TiO2/biochar materials.

the clear presence of a distinctive anatase phase in all TiO2based biochar materials (Table 1). It is also worth noting the existence of a sharp peak at 26.6° in the case of TiO2/MSP700 corresponding to SiO2 −quartz reflection. All materials exhibited almost identical XRD patterns differing only in terms of the intensity of the anatase diffraction lines. Some researchers claim that TiO2 anatase phase exhibits lower rates of recombination e−/h+ and higher adsorptive affinity for organic compounds compered to TiO2 rutile phase.3,5,7 In order to check the role of ultrasound, TiO2/SWP700 “in silence” was prepared (Table 1). Surprisingly, thus prepared material has different crystallite size (18 nm) than TiO2/ SWP700 prepared by the ultrasound-assisted wet impregnation method (22 nm). The cause for such differences may be owing to the intimate contact of two phases through a heterostructure vicinity. This new discovery may help to understand the roles of ultrasound in the formation of this kind of materials with photocatalytic properties. The XRF technique is a powerful tool for chemical analyses of many major and trace elements in prepared photocatalysts. It sometimes happens that trace elements (fingerprints) were observed in the elemental maps, e.g., Rh, Fe, and Cu, from the X-ray source scatter. Additionally, it has been shown by the XRF technique that the main elements for biochar-based photocatalysts (results not shown) are Ti (∼97%) and Ca (1.3 ± 3.1%). In the case of TiO2/MSP550 and TiO2/MSP700 we also detected potassium (5.6% and 7.6%, respectively) and silicon (14% and 8.8%, respectively). To calculate the band gap energy, revealing the light harvesting ability of the resultant biochar-supported catalysts, the samples were subjected to UV−vis diffuse reflectance spectroscopy measurements (Figure 5). The absorption edge is found to shift toward longer wavelengths as observed for TiO2/ biochar photocatalysts (Table 1). On the other hand, TiO2 P25 Evonik showed a clear absorption edge at around 388 nm corresponding to a band gap of ∼3.20 eV. The Kubelka−Munk function was developed to calculate the band gap for TiO2/ MSP550, TiO2/MSP700, TiO2/SWP550, and TiO2/SWP700 and found to be 2.43, 2.50, 2.45, and 2.12 respectively (Table 1). The most remarkable differences in the band gap energy values among all prepared materials are observed for TiO2/ SWP700 composite which in turn could be further extended to longer wavelength. Therefore, the improvement in the light absorption and the extended absorption edge may be responsible for the role of SWP700 in the hybrid nanostruc-

Figure 3. SEM images of TiO2/biochar materials.

The images show irregular plate-like structure, which may be favorable for adsorption and photoreactions. Therefore, good organic contaminants adsorption performance can be expected. Furthermore, obvious pores resulting from the slits among the plate-like particles can be also observed (observations also corroborated by BET characterization). X-ray powder diffraction (XRD) techniques were carried out in order to evaluate the structural characteristics of TiO2/ biochar materials. XRD patterns compiled in Figure 4 point out 6276


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Table 1. Results of Crystallite Size, Crystal Phase, Band Gap Energy, and Absorption Threshold over All Tested Photocatalysts XRD

UV−vis

photocatalyst

crystallite size (nm)

crystal phase

Egap (eV)

absorption threshold (nm)

TiO2/MSP550 TiO2/MSP700 TiO2/SWP550 TiO2/SWP700 TiO2/SWP700_Without_US

18 23 12 22 18

anatase anatase anatase anatase anatase

2.43 2.50 2.45 2.12 2.74

515 498 508 586 455

Figure 5. Diffuse reflectance UV−vis spectra of TiO2/biochar materials and TiO2 P25.

Table 2. XPS Binding Energies and Atomic Ratio for All Prepared Composites Prepared by Ultrasound-Assisted Wet Impregnation Method photocatalysts C 1s BE eV (atom %)

Ti 2p BE eV (atom %) O 1s BE eV (atom %)

C−C sp2 C−C sp3 C−O−C COOH OC−O, CC−OH Ti4+ Ti3+ Ti−O−Ti Ti−O C−O, C−OH (C−O−C)

TiO2/MSP550

TiO2/MSP700

TiO2/SWP550

TiO2/SWP700

283.7 284.8 286.1 287.4

(5.19) (2.19) (0.22) (0.08)

283.9 284.8 285.9 287.6

(6.19) (2.82) (1.45) (0.33)

460.9 459.5 529.5 530.6 532.8

(15.10) (0.21) (20.06) (26.69) (13.86)

461.1 460.0 529.8 530.7 533.0

(14.52) (0.48) (26.01) (18.71) (14.73)

283.8 284.8 285.8 287.3 288.5 460.7 459.6 529.2 530.7 532.4

283.9 284.9 286.0 287.6 288.8 461.1 459.6 529.4 530.3 532.4

(32.0) (2.82) (3.47) (3.47) (2.18) (8.49) (0.57) (22.63) (8.84) (5.39)

(30.11) (7.50) (3.63) (2.99) (1.96) (11.71) (0.66) (21.7) (12.33) (4.87)

knowledge that the physical effects of ultrasound can influence the physical and functional properties of materials due to the shear forces generated during acoustic cavitation. It can be stated that TiO2 (in TiO2/SWP700) prepared in “in silence” has a higher band gap energy (2.74 eV) than TiO2 (in TiO2/ SWP700) prepared by the ultrasound-assisted wet impregnation method (2.12 eV). Thus, it can be concluded that ultrasound can act as an “interfacial mediator” in order to improve the visible light photoabsorbability of TiO2 in TiO2/ SWP700 composite, while the intimate interfacial contact between SWP700 and TiO2 may be still retained. HR XPS surface analysis techniques were performed to evaluate the chemistry at the surface of TiO2/biochar materials. The oxidation states and atomic concentrations of Ti 2p, O 1s, and C 1s of all TiO2/biochar materials as obtained by XPS are given in Table 2. In all prepared materials the dominant bands of Ti 2p are located at binding energies of 461.0 ± 0.2 and 459.7 ± 0.2 eV and clearly correspond to Ti4+ and Ti3+ in TiO2

tures. These observations can suggest an increase of the surface electric charge of SWP700, which can lead to modifications of the fundamental process of e−/h+ pair formation while applying visible irradiation. In addition, introduction of a local trap state corresponding to Ti3+ in TiO2/SWP700 (Table 2) can improve the transfer efficiency of electrons between biochar and TiO2, which will enhance the photocatalytic redox reactions (Ti4+ to Ti3+). The presence of excess Ti3+ and oxygen vacancy sites in TiO2/SWP700 can cause the visible-light absorption due to generation of Ti3+ ions in the band gap just below the conduction band (CB) of TiO2 which lead to a band gap decrease.21 Furthermore, TiO2/SWP700 is considered to take benefit of the SWP700 ability to absorb visible light to initiate the photocatalytic reaction, leading to formation of some reactive oxidative species. To investigate the specific role of ultrasound on the optical properties, our best performing TiO2/SWP700 photocatalyst “in silence” was prepared (Table 1). It is fairly common 6277


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Figure 6. Raman spectra of SWP700 and TiO2/SWP700 prepared by the ultrasound-assisted wet impregnation method and pure SWP700 carbon materials (graphite and graphene oxide). (Inset) TiO2/SWP700 prepared without ultrasound.

structure.20−24 In addition, TiO2/SWP700 exhibited the highest atomic concentration (0.66 atom %) of Ti3+ and very high sp2/ sp3 ratio (4.0) compared with the other photocatalysts. Generally, Ti3+ species in the TiO2 structure are very important for the heterogeneous photocatalysis since they can trap the photogenerated electrons and leave unpaired charges behind to improve the photocatalytic activity.21 It is also worth pointing out that increasing the Ti3+ density promotes effective segregation of electrons and interface charge transfer and then increases the photocatalytic performance.21−25 According to the results obtained by the XPS studies, it can be stated that the O/ Ti ratios for some samples (especially TiO2/SWP700) are slightly below (1.97) the stoichiometric value (O/Ti = 2.0). On the basis of the results obtained, the presence of surface oxygen vacancies is expected for this composite. Formation of oxygen vacancies on TiO2/SWP700 may induce formation of unpaired electrons or Ti3+ centers in order to maintain charge balance. It should be noted that surface Ti3+ defects can interact with O2 molecules (i.e., a nondissociative adsorption of O2 molecule on vacancy defect site), producing the superoxide species, O2•−.23 Interestingly, the presence of surface Ti3+ defects can enhance the photocatalytic activity in view of inhibition of the electron− hole recombination23,27 and increased oxygen adsorption.23,27 This is mainly due to the fact that ultrasound as an “interfacial mediator” can significantly improve the intimate interfacial contact between TiO2 precursor and SWP700. The strong bands at 530.6 ± 0.2 and 529.5 eV ± 0.2 eV are related to the Ti−O and Ti−O−Ti bonds of TiO2, which suggest that the chemical state of oxygen is the main lattice oxygen in TiO2.28,29 The band at 532.7 eV ± 0.3 eV corresponds to the O−H bond (hydroxyl group), C−O bond, and H2O adsorbed on the surface of titania.30,31 The peak of C 1s at around 281−282 eV arising from the Ti−C bond was undetectable, strongly suggesting that carbon does not substitute for the oxygen atom in TiO2 anatase lattice.30,31 Raman spectroscopy was carried out to provide information about TiO2/SWP700 and study the TiO2 form and the carbon matrix (Figure 6). The spectra showed representative Raman modes (144 cm−1 (Eg), 198 cm−1 (Eg), 394 cm−1 (B1g), 514 cm−1 (B1g + A1g), and 634 cm−1 (Eg)) corresponding to the characteristic peaks of the TiO2 anatase phase which was also

confirmed by HR-TEM (Figure 7), from which it can be seen that TiO2 anatase nanoparticle may uniformly carpet the

Figure 7. HRTEM micrograph of TiO2/SWP700.

SWP700 surface, suggesting the intimate interfacial contact between TiO2 and SWP700. It is worth emphasizing the peak position (in particular, the 144 cm−1 Eg mode) and the peak width (i.e., full width at half-maximum are in agreement with literature data reported for TiO2 anatase nanocrystals).32,33 No peaks related to either rutile or other TiO2 polymorphs were observed for TiO2/SWP700 (confirmed also by XRD). Figure 6 also shows the Raman spectra containing a defect-derived D (disorder) peak centered roughly at ∼1350 cm−1 that is related to the presence of sp3 C−C atoms and a G (graphitic) peak at ∼1595 cm−1 that is related to the E2g phonon of the C sp2 C−C atoms.34,35 It is worth noting that the intensity ratio of the D band to G band (ID/IG ratio), which is important for indicating the amount of functionalization in a carbon material. A high ID/IG 6278


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phase was a Milli-Q water−methanol (65:35 v/v) mixture with 0.1% of CF3COOH at a flow rate of 1 mL min−1. The injection volume was 10 μL. Blank experiments were performed in the dark as well as with illumination and no catalyst, without observable change in the initial concentration of phenol in both cases. To investigate the effects of active species generated during the photocatalytic reaction using TiO2/SWP700 under UV and visible illumination, free radical capture experiments were conducted.36,37 The three major oxidants involved in photodegradation of organics in water, that is, hydroxyl radical (OH•), hole (h+), and superoxide radical (O2•−), were trapped by adding 0.5 mL of tert-butanol (t-BuOH), 0.1 mM of ammonium oxalate (AO), and 0.5 mM of 1,4-benzoquinone (BQ), respectively, into the phenol solutions (see Supporting Information, Figure S3). The photocatalytic activity of all prepared TiO2/biochar composites have been evaluated considering the phenol degradation under UV irradiation (Figure 8). Negligible phenol

ratio means a high degree of disorder in the carbon network.34,35 If ID/IG > 1, the structure is more disordered. The intensity ratio of the D band to G band (ID/IG) pointed out disorder in carbon materials such as pure SWP700 (ID/IG = 0.48) and SWP700 treated by the ultrasound-assisted wet impregnation method (ID/IG = 0.71) which may be caused by structural surface defects or disorder within the carbon network, which indicates the difference in microstructures and SWP700 treated by the ultrasound-assisted wet impregnation method had fewer surface defects than pure SWP700. It is worth mentioning that TiO2/SWP700 prepared by the ultrasound-assisted wet impregnation method showed a slightly lower value (ID/IG = 0.37) compared to pure SWP700 (ID/IG = 0.48). This decrease might be caused by reconstruction of structural defects within the sp2 carbon network that arose upon the ultrasound-assisted wet impregnation method and may suggest the existence of a conduction network throughout the TiO2/ SWP700 composite, connecting TiO2 particles to the conducting SWP700 support. To check the role of ultrasound, TiO2/SWP700 without ultrasound was prepared, and in Figure 6 (inset) the main features of the spectra of this TiO2/SWP700 photocatalyst show noticeable variation and without the presence of D and G bands due to a strong fluorescence interference which may suggest high recombination of electrons and holes. It can be stated that ultrasound can be a “structuredirecting factor” for the prepared hybrid material, and the intimate interface connection between SWP700 and TiO2 can clearly be observed only in the presence of ultrasound as interfacial mediator, indicating that TiO2 particles were well attached on the SWP700 surface. Photocatalytic Activity in Aqueous Phase. All photocatalytic reactions were carried out in a Pyrex cylindrical double-walled immersion well photoreactor. The bath photoreactor was stirred magnetically to obtain a homogeneous suspension of the catalyst. A medium-pressure 125 W mercury lamp (λmax = 365 nm), supplied by Photochemical Reactors Ltd. UK (Model RQ 3010), was placed inside the glass immersion well as the light irradiation source. The reaction temperature was set at 30 °C. Phenol solution (50 ppm) was prepared in Milli-Q water. Experiments under UV light were performed from 150 mL of the mother solution, and 1 g L−1 of photocatalyst concentration was used after previous optimization. Photocatalytic degradation under visible light was investigated in a glass reactor (V = 20 mL) by means of sunimitating superquiet xenon lamp (150 W, L2195 Hamamatsu, 240−2000 nm) with a UV/IR-Cut filter (BAADER, blocks UV below 400 nm and IR above 680 nm). The average luminous intensity for UV light (220−400 nm, ∼264.3 W m−2) and visible light (distance between the lamp and the photoreactor = 8 cm, 400−680 nm, ∼60.6 W m−2) was examined by a radiometer HD2302.0 (Delta Ohm, Italy). After optimization studies, the optimal adsorption−desorption equilibrium time of 40 min under dark condition between the photocatalyst and phenol was selected (see Supporting Information, Figure S2). At each sampling point, approximately 1 mL of sample was periodically taken from the photoreactor and filtered through 0.2 μm, 25 mm nylon filters to remove photocatalyst. Phenol degradation was measured, after external standard calibration, by high-performance liquid chromatography (HPLC, Waters model 590 pump) equipped with a Dual Absorbance Detector (Waters 2487). Separation was performed on an XBridge C18 5 μm 4.6 × 150 mm column provided by Waters. The mobile

Figure 8. Photocatalytic phenol degradation under UV and visible light over all tested photocatalysts after 240 min of illumination.

degradation ( TiO2/MSP700 > TiO2/NORIT > TiO2/ SWP550 > TiO2/MSP550 > TiO2/SWP700_without_US > TiO2 P25/SWP700_mech.mix. It is worth emphasizing that titanium leaching after the photocatalytic tests was not noticed in the aqueous solution (confirmed by inductively coupled plasma mass spectrometry (ICP-MS) and XRF, see Supporting Information: Figure S4) for the best performing TiO2/SWP700 photocatalyst prepared by ultrasound-based procedures. Moreover, it was shown that the photocatalyst TiO2/SWP700 prepared without ultrasound suffered from leaching of Ti (0.1 wt % leaching detected by ICP-MS) in the aqueous solution after 240 min. 6280


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products were quantitatively analyzed by means of online gas chromatography (HP 5890 series II Hewlett-Packard USA equipped with a flame ionization detector (FID) and a methanizer model 510 instrument supplied by SRI INSTRUMENTS) and identified offline by GC-MS (HP-5 column GC (6890 Series)-MS(5973) Hewlett-Packard equipped with FID). To understand better the impact of gas-phase reactions on all synthesized materials, selective photooxidation of methanol was performed. After 240 min of illumination, CO2 and methyl formate (MF) were detected as the only two reaction products by using eqs 1 and 2

aqueous phase. It was observed that TiO2/SWP700 showed the highest COD removal with 83.8% (UV light) and 81.4% (visible light), while for SWP700 it was only 12.0% (UV light) and 12.8% (visible light) (see Supporting Information: Table S4). These data show that TiO2/SWP700 composite photocatalyst does not have a considerable difference in COD removal under UV and visible light (the significant difference is in phenol degradation). It should be noted that when the phenol content during photodegradation of phenol is decreased, more active site and •OH radicals become available to react with phenol and hazardous byproducts. In the intermediate phase, byproducts were generated from direct oxidation of phenol including aromatic compounds and hydrocarbon chains. All these intermediates can eventually be oxidized to CO2 via various subintermediates following the ring-opening processes.2,6−8,14 On the other hand, another fraction of substrates adsorbed onto SWP700 and without any contact with TiO2 undergoes photodegradation by attack of reactive oxygen species that are generated on the surface of the TiO2 material but migrates onto the surface of biochar. It is believed that the ROS (reactive oxygen species) may diffuse over submillimeter distances from the surface of TiO2, so some ROS can reach the target compound situated on the surface of the biochar, where most of the phenol remains adsorbed. The reuse of any photocatalyst is very important for its water treatment application. In order to assess the long-term performance of the composite photocatalyst, a recycling test for phenol degradation was carried out. After each cycle, the photocatalyst was filtered out and left to dry at 110 °C overnight before use in the next cycle. To evaluate the stability and reusability of our best photocatalytic material (TiO2/ SWP700), successive cycles of the photocatalytic phenol degradation were carried out (Figure 9) under the same reaction conditions with the loss of only ∼10% of phenol degradation activity after run 5. The results revealed that the photocatalytic activity of TiO2/SWP700 photocatalyst has promising cyclic stability. Photocatalytic Activity in the Gas Phase. The schematic view of the photocatalytic reaction system in the gas phase is shown in Figure 10. After achieving reagent−photocatalyst adsorption equilibrium in the dark (after 2 h), liquid methanol was fed into the photocatalytic reaction system by a programmable syringe pump (NE-1000, Syringe Pump Co.) at a constant flow rate of 1.5 μL min−1. The whole reactor system lines were heated to prevent condensation. This gas mixture containing 0.9 vol % of methanol and 99.1 vol % of air was supplied at a flow rate of 25 cm3 min−1 into the photoreactor. The gas flow rates were measured and controlled by mass flow controllers (Bronkhorst HI-TEC). The continuous fixed-bed photoreactor was vertically enclosed by an aluminum foil cylindrical reflector (20 cm × 13 cm × 1 mm) to exclude any external light source and maximize light energy usage within the reactor. The light source was a mediumpressure 125 W mercury lamp (λmax = 365 nm), supplied by Photochemical Reactors Ltd. UK (Model RQ 3010) and sunimitating xenon lamp (150 W, L2195 Hamamatsu, 240−2000 nm) with a UV/IR-Cut filter (BAADER, blocks UV below 400 nm and IR above 680 nm) built into a lamp housing and centered vertically in the reflector (2.5 cm between the lamp and photoreactor) and thermostated at 30 °C. The average luminous intensity for UV light (220−400 nm, ∼274.9 W m−2) and visible light (400−680 nm, ∼271.5 W m−2) was examined by a radiometer HD2302.0 (Delta Ohm, Italy). Reaction

2CH3OH + 3O2 → 2CO2 + 4H 2O

(1)

2CH3OH + O2 → HCOOCH3 + 2H 2O

(2)

Photocatalyst efficiency was calculated according to methanol conversion (X), yield (Y), and selectivity (S) of methyl formate on the basis of eqs 3, 4, and 538 Conversion (X) of CH3OH X(%) =

(mol of CH3OHin − mol of CH3OHout) mol of CH3OHin

× 100 (3)

Yield (Y) of HCOOCH3 Y(%) =

2 × mol of HCOOCH3products mol of CH3OHin

× 100 (4)

Selectivity (S) of HCOOCH3 S(%) =

mol of HCOOCH3products mol of (HCOOCH3products+CO2 )

× 100 (5)

Starting with the control tests in the gas phase, two tests were done: (1) photocatalytic reaction in the gas phase (UV lamp and air flow) without photocatalyst and (2) photocatalytic reaction in the gas phase (no UV light, air flow, and at a temperature up to 100 °C) in the presence of photocatalyst (the results of these experiments are not presented here). Photolysis (without photocatalyst) revealed negligible conversion of methanol (∼5%). In a thermal test up to 100 °C, TiO2/SWP700 photocatalyst was not active. It should be also noted that GC online analysis did not show any leaching of carbon (SWP700) during the photocatalytic test in the gas phase for the best performing TiO2/SWP700 photocatalysts (stability test air flow (25 cm3 min−1), UV illumination, and absence of methanol). Figure 11 shows the results photo-

Figure 11. Photo-oxidation of methanol in the gas phase over all tested photocatalysts after 240 min of illumination. 6281


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region was not clearly resolved (no functional groups), the spectra of TiO2/SWP700 after illumination in the gas and liquid phases showed additional peaks. The broad peak in the 3200−3600 cm−1 region suggests that the TiO2/SWP700 surface is rich in hydroxyl groups, which can be evidence for the existence of Ti−OH.30,39−41 For this reason, H2O are easily adsorbed on the surface of this high BET of biochar-based photocatalyst. The main peaks appearing in the range 2900− 3000 cm−1 correspond to methoxy species (νs(CH3) and 2δ2(CH3) of CH3 in adsorbed OCH3).42−48 Bands observed in the 1730−1780 and 1600−1650 cm−1 regions correspond to CO vibrations of COOH, OC−O, and CC− OH.39,40,43,45−48 The bands in the range 450−550 cm−1 were assigned to Ti−O vibration.41,49,50 In addition, the FT-IR bands in 700−900 cm−1 pointed out the Ti−O−Ti bond in the TiO2 anatase phase.41,43−50 After UV illumination of TiO2/SWP700 in the liquid phase (Figure 13, TiO2/SWP700_AR_liquid phase), peaks appearing in the range 2400−2500 and 750−900 cm−1 correspond to gaseous CO240,46−53 and the abovementioned Ti−O−Ti band, respectively. In the case of TiO2/ SWP700 after UV illumination in the gas phase (Figure 13, TiO2/SWP700_AR_gas phase_MeOH), new bands appeared and correspond to gaseous CO2 associated with total oxidation of methanol (2400−2500 cm−1) and C−O band from methanol (1020−1070 cm−1)42,46,47 which appear during photocatalytic oxidation of methanol in the gas phase. Moreover, new bands at 1360, 1380 (νs(OCO)), and 1580 cm−1 (νas(OCO)) may correspond to adsorbed formate species.46−48,54 The HR XPS spectrum (Table S5: Supporting Information) of C 1s indicates that the surfaces of SWP700 and TiO2/ SWP700 treated by the ultrasound-assisted wet impregnation method have strong bands at 287.3 ± 0.3 and 288.6 ± 0.2 eV clearly corresponding to COOH, OC−O, and CC−OH in comparison with pure SWP700 and TiO2/SWP700 (without ultrasound) which do not have OC−O and CC−OH surface functional groups and weaker bands corresponding to COOH in the case of TiO2/SWP700 (without ultrasound). These results confirm that the presence of surface functional groups favored deposition of TiO2 on biochar (SWP700) surface and reveal a better interaction between TiO2 precursor and SWP700. The presence of these functional groups might be necessary to improve the dispersion of TiO2 where the carboxylic groups can play a significant role as anchoring groups for titanium tetraisopropoxide (TiO2 precursor). On the basis of the experimental evidence presented in Table S5 (see SI), we can state that there was a significant increase in C−C sp2 atoms in the presence of TiO2/SWP700 after photocatalytic reaction in liquid (16.31 atom %) and gas phase (2.18 atom %) with a remarkable decrease of TiO2/SWP700 prepared “in silence” (20.5 atom %). Such results show the important effect of sonication as an “interfacial mediator” on the C−C sp2 surface functional groups which can significantly improve the intimate interfacial contact between SWP700 and TiO2. It is well known that C−C sp2 surface functional groups can act as scattering centers which may provide exceptional electrical and optical properties, high resistivity, and electron mobility in carbon materials.55−58 The measured photoluminescence (PL) emission spectra of SWP700 and TiO2/SWP700 prepared by ultrasound-based procedures and TiO2/SWP700 prepared without ultrasound are presented in Figure S5 (Supporting Information). It is well known that the lower photoluminescence emission spectra

oxidation of methanol over all TiO2/biochars together with that of the TiO2/NORIT photocatalyst for comparison. Additionally, none of the pure biochar was active in the photo-oxidation of methanol. It is worth emphasizing that the TiO2/SWP700 photocatalyst achieved the highest conversion of methanol (∼90%) of all tested composites, high methyl formate production (∼80%), and high yield of methyl formate (∼88%) after 240 min of illumination. After changing the light source to the visible range, all hybrid materials exhibited lower methanol conversion ( TiO2/ SWP550 > TiO2/NORIT > TiO2/MSP700 > TiO2/MSP550. The study of the reusability and sequential application of synthesized photocatalysts is an important factor for future applications, and therefore, with this in mind, the cyclic performance of TiO2/SWP700 was investigated in five consecutive runs of photo-oxidation of methanol. The results of the catalytic long-run stability tests are shown in Figure 12.

Figure 12. Photocatalytic activity of TiO2/SWP700 in long-run test of photocatalytic oxidation of methanol in the gas phase.

During this experiment, after each run of 240 min of illumination, the TiO2/SWP700 photocatalyst was “cleaned“ in a 25 mL min−1 flow of air at room temperature to remove all physisorbed reagents and products from the photocatalyst surface (monitoring by GC online analysis), and then the photocatalyst was used for the following run (lamp “on”). It should be emphasized that our best photocatalytic material (TiO2/SWP700) undergoes successive cycles of the photooxidation of methanol under the same reaction conditions with a loss of only ∼10% of photocatalytic conversion after run 5 (20 h of illumination). Tentative Oxidative Transformations Pathways of the Best Performing Photocatalyst (TiO2/SWP700) in Aqueous and Gas Phases. FT-IR is very informative in the determination of various functional groups on TiO2/SWP700 (Figure 13) materials before and after photocatalytic phenol degradation (liquid phase) and selective oxidation of methanol (gas phase). While the FTIR spectrum of pure SWP700 in this 6282


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Figure 13. FTIR spectra for SWP700 and TiO2/SWP700 photocatalyst. Suffix AR stands for after reaction.

point out the lower recombination rate of photogenerated e−− h+ pairs, which leads to the high photocatalytic activity of semiconductor photocatalysts.59−61 Consequently, the very low photoluminescence emission spectra for TiO2/SWP700 prepared by the ultrasound-assisted wet impregnation method points out that the photocatalytic efficiency of TiO2 may be improved owing to the intimate contact of the TiO2 and SWP700 through a heterostructure vicinity. As a result of excitation irradiation, biochar as an electron collector and transporter can benefit the charge transfer in the TiO2/ SWP700 and inhibit charge recombination. It is generally accepted that the PL spectrum with low intensity indicates efficient separation of the charge carriers, leading to participation of more electrons and holes in the oxidation and reduction reactions. However, there is a considerable decrease in the intensity of the PL spectrum for TiO2/SWP700 prepared by the ultrasound-assisted wet impregnation method compared to that of TiO2/SWP700 prepared “in silence”. These observations suggest the formation of a heterojunction formed at the interface between TiO2 and SWP700 in the presence of ultrasound, which confirms an excellent electron− hole separation efficiency of the composite material; hence, enhanced photocatalytic performance can be acquired in this study. Tentative Degradation Pathway of Photocatalytic Reaction in Aqueous Phase. On the basis of the experimental evidence presented, and under our reaction conditions we can state that our best photocatalyst (TiO2/ SWP700) achieved 64.1% (UV light) and 33.6% (visible light) of phenol degradation. In order to gain further insight into the photocatalytic mechanism, trapping experiments of active species were performed to observe the active species generated during the photocatalytic process. It is observed that h+, •OH, and •O2− may be involved in photocatalytic degradation of phenol under UV and visible illumination (SI, Figure S3). In addition, TiO2/SWP700 photocatalyst can efficiently transfer photoinduced electrons to reduce the recombination of e−−h+ pairs which are trapped or react in aqueous phase producing reactive species such as •OH and •O2−. •OH radicals were formed on the surface of TiO2/SWP700 by the reaction of holes with adsorbed water and/or surface functional groups like Ti−OH and Ti−O−Ti (based on FTIR (Figure 13) and XPS

which showed a lower atom % of Ti−O−Ti after reaction (SI, Table S5)), which may be attributed to enhanced efficiency in the photodegradation of phenol. Additionally, hydroxyl groups on the surface of TiO2/SWP700 can be responsible for the reaction of adsorbed water with titania and creation of Ti−OH according to reaction 6 H 2O + Ti−O−Ti → 2Ti−OH

(6)

TiO2/SWP700 with the highest surface Ti3+ species (0.66 at %), compared to other photocatalysts, can reduce the recombination between trapped electrons Ti3+ and trapped holes (Ti−OH and Ti−O−Ti), and thanks to interfacial interaction of SWP700 and TiO2, more electrons and holes were transferred. Additionally, the excellent support of SWP700 can act as an electron mediator to the photogenerated electrons of TiO2 from the conduction band (CB) with a quick transfer to the surface of SWP700 and then reaction with O2 absorbed on its surface or dissolved in water to produce •O2− by photogenerated electrons. Meanwhile, OH− or H2O can be oxidized to hydroxyl radicals (•OH) by the photogenerated holes, and the •OH radicals and/or Ti−OH and Ti−O−Ti can directly attack the phenyl ring of phenol, yielding degradation products including aromatic compounds and hydrocarbon chains, and subsequently, these intermediates were completely mineralized to carbon dioxide and water.13,14,62−64 Therefore, h+, •OH, and •O2− (all of them observed in the photocatalytic activity) function as ROS in the photocatalytic reactions. To further understand the potential role of molecular oxygen in the complex photocatalytic mechanism, photocatalytic reactions of phenol in the absence of oxygen (argon instead of air) were carried out. It should be noted that our best performing photocatalyst TiO2/SWP700 photocatalyst in the absence of oxygen resulted in phenol degradation of 36.1% (UV light) and 18.4% (visible light). It is necessary to emphasize that the reaction of molecular oxygen with photogenerated electrons for the generation of •O2− is important for photodegradation of phenol over TiO2/ SWP700. From this point of view, a relatively high surface area (399 m2g−1), 100% anatase phase and strong adsorption ability for phenol, the relatively narrower band gap of TiO2/ SWP700 composites (2.12 eV) and more harvesting of visible light owing to the presence of SWP700, and lower 6283


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ACS Sustainable Chemistry & Engineering recombination rate of the photoexcited e−−h+ pairs prolong the lifetime of a photongenerated carrier, thus greatly improving the photocatalytic efficiency and stability of TiO2/SWP700 composites in the removal of phenol. Finally, it should be noted that the presence of a higher percentage of C−C sp2 surface functional groups (confirmed by HR-XPS) on the TiO2/ SWP700 may be also attributed to enhanced efficiency in the photocatalytic reaction in the aqueous phase. Tentative Oxidation Pathway of Photocatalytic Selective Reaction in the Gas Phase. To get some insights into the plausible reaction pathway of the photocatalytic reaction in the gas phase, a set of FT-IR measurements of TiO2/SWP700 was carried out (Figure 13). During light irradiation, methanol is converted to a greater extent and FT-IR band intensity associated with adsorbed intermediate methoxy species, hydroxyl species, formate species and gaseous CO2. The presence of adsorbed formate molecules during oxidation of methanol to methyl formate is not uncommon, and these have been suggested to be due to formation upon reaction of surface oxygen with adsorbed formaldehyde.54,65,66 It should be also noted that photocatalytic methanol photo-oxidation would be expected if significant UV radiation was incident on the TiO2/SWP700 photocatalysts, and as a consequence, the surface hydroxyl groups on prepared composite might also play a vital role in the photocatalytic reaction, as these groups can inhibit the recombination of photogeneration charges.1,3,22−25 It is well known that methyl formate is formed by the reaction of adsorbed formaldehyde and adsorbed methoxy species, which is reported to take place when the two are in close proximity on the photocatalyst surface.67,68 Nevertheless, formate molecules on the catalyst surface can react with methanol69 to form methyl formate as an alternative route, suggesting the importance of formate species during the reaction. The presence of formate species on the surface (which are available to react with methanol) is confirmed on the photocatalyst surface during the photo-oxidation of methanol in the frame of the present research work. Surface adsorbates, particularly formate species, build up on the photocatalyst surface of TiO2/SWP700 during the photo-oxidation reaction and enhanced the photo-oxidation of methanol. On the other hand, O2 adsorbed on the surface of TiO2/SWP700 may accept the electron and form the •OH radical,70,71 which oxidizes the adsorbed molecules directly on the surface. The same mechanism could explain the enhanced photocatalytic efficiency of TiO2/SWP700 composites tested for the photocatalytic oxidation of methanol in the gas phase. Mo and Ye71 reported that various oxygen species like crystal lattice oxygen (O2−) (∼528.8 eV), hydroxyl oxygen (O−) (∼530.6 eV), and adsorbed oxygen (O2−) (∼532.1 eV) may appear, which was confirmed by XPS analysis in our case. On the basis of experimental evidence presented in Table S5 (see SI), we can state that there was a significant increase in oxygen surface species such as crystal lattice oxygen (O2−) (0.31 atom %), hydroxyl oxygen (O−) (0.84 atom %), and adsorbed oxygen (O2−) (3.52 atom %) in the presence of TiO2/SWP700 after photocatalytic oxidation in the gas phase. Not only adsorbed oxygen species (O2−) but also surface hydroxyl oxygen species (O−) were very favorable for the photocatalytic oxidation reaction.26,72,73 Nevertheless, the hydroxyl oxygen species (O−) could produce active species (•OH free radical). Consequently, the adsorbed oxygen species (O2−) on the surface and their adsorption and diffusion properties can be dominant for the transportation process of various oxygen species in the

photocatalytic oxidation26,74 and could be responsible for photocatalytic oxidation of methanol to methyl formate in the presence of TiO2/SWP700. It is interesting that the peroxyl radicals (•O2) generated after this electron capture could accelerate oxidation of organic substances, and the adsorbed oxygen species (O2−) can serve as the capturer or taker of photogenerated electrons.73,74 Additionally, the adsorbed O2 also served as an oxidant, which can control the recombination of photo induced electron−hole recombination.74 Consistently, the observed increase in adsorbed oxygen species on the photocatalyst surface could be responsible for the enhanced activity in the selective oxidative esterification of methanol to methyl formate in continuous gas phase (high activity (∼90%), high selectivity to methyl formate (∼80%), and high yield of methyl formate (∼88%)) after 240 min of illumination under UV light. The results may confirm that coupling of TiO2 with SWP700 by a good interfacial contact significantly facilitates the charge carrier transfer process upon ultraviolet illumination. Concurrently, this is the first direct and robust experimental evidence to show the role of SWP700, which can behave as an electron reservoir to capture or shuttle photogenerated electrons from the semiconductor upon light irradiation in selective photo-oxidation of methanol in the gas phase.

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CONCLUSIONS This paper was aimed to illustrate a low-cost, efficient, and environmentally friendly approach to synthesize and significantly improve the photoactivity of titania-based wood (Soft Wood Pellets (SWP)) and straw-derived (Miscanthus Straw Pellets (MSP)) composites. This was achieved by using an ultrasound-assisted methodology that promoted formation of intimate interfacial contact between biochar and TiO2. Such interfacial composition chemistry not only optimizes the photogenerated charge carrier transfer pathway across the interface between biochar and TiO2 but also efficiently improves the lifetime/transfer of charge carriers (low recombination rate) in the heterostructure TiO2/biochar systems (especially TiO2/SWP700) but also significantly influences the physicochemical properties of prepared hybrid materials. The best performing TiO2/SWP700 photocatalyst exhibited favorable properties, such as 100% anatase nanoparticles, visible light absorption and high surface area, and enhanced photocatalytic activity/selectivity in liquid and gas phases. Importantly, TiO2/SWP700 was the most photocatalytically active (without Ti leaching) in phenol photodegradation (UV light 64.1%, visible light 33.6%) and photocatalytic selective reaction with extraordinarily high activity (∼90%), high selectivity to methyl formate (∼80%), and high yield of methyl formate (∼88%) after 240 min of illumination. Obtained results revealed the existence of an intimate contact between SWP700 and TiO2 phases in the composite photocatalysts capable of attaining unique electron transfer properties on the resulting composites. Thus, it can be concluded that ultrasound can act as a “interfacial mediator”, may lead to improved visible light photoabsorbability of TiO2/ SWP700 composite, and promotes creation of an intimate heterojunction formed at the interface between TiO2 and SWP700 in the presence of ultrasound and by this explaining the excellent electron−hole separation efficiency of the composite material. This work offers a simple, economical, and powerful tool to prepare TiO2/biochar photocatalysts with high photocatalytic activity and stability. This is an important step ahead in development of viable applications of photo6284


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(8) Liu, W. J.; Jiang, H.; Yu, H. Q. Development of biochar-based functional materials: toward a sustainable platform carbon material. Chem. Rev. 2015, 115, 12251−12285. (9) Kim, J. R.; Kan, E. Heterogeneous photocatalytic degradation of sulfamethoxazole in water using a biochar-supported TiO2 photocatalyst. J. Environ. Manage. 2016, 180, 94−101. (10) Matos, J. Eco-friendly heterogeneous photocatalysis on biocharbased materials under solar irradiation. Top. Catal. 2016, 59, 394−402. (11) The Inside Story: A Guide to Air Quality, U.S. Environmental Protection Agency, Office of Radiation and Indoor Air (6609J): Washington, DC, 2009. (12) Mo, J.; Zhang, Y.; Xu, Q.; Lamson, J. J.; Zhao, R. Photocatalytic purification of volatile organic compounds in indoor air: A literature review. Atmos. Environ. 2009, 43, 2229−2246. (13) Lin, S. H.; Juang, R. S. Adsorption of phenol and its derivatives from water using synthetic resins and low-cost natural adsorbents: A review. J. Environ. Manage. 2009, 90, 1336−1349. (14) Grabowska, E.; Reszczynska, J.; Zaleska, A. Mechanism of phenol photodegradation in the presence of pure and modified-TiO2: A review. Water Res. 2012, 46, 5453−5471. (15) Colmenares, J. C. Sonication-induced pathways in the synthesis of light-active catalysts for photocatalytic oxidation of organic contaminants. ChemSusChem 2014, 7, 1512−1527. (16) Sathishkumar, P.; Mangalaraja, R. V.; Anandan, S. Review on the recent improvements in sonochemical and combined sonochemical oxidation processes − A powerful tool for destruction of environmental contaminants. Renewable Sustainable Energy Rev. 2016, 55, 426−454. (17) Colmenares, J. C.; Lisowski, P.; Łomot, D. A novel biomassbased support (Starbon) for TiO2 hybrid photocatalysts: a versatile green tool for water purification. RSC Adv. 2013, 3, 20186−20192. (18) Wang, G.; Cheng, B.; Zhang, J.; Xu, L.; Yin, T. Facile Synthesis and photocatalytic property of titania/carbon composite hollow microspheres with bimodal mesoporous shells. Int. J. Photoenergy 2012, 2012, 1−9. (19) Coromelci-Pastravanu, C.; Ignat, M.; Popovici, E.; Harabagiu, V. TiO2-coated mesoporous carbon: Conventional vs. microwaveannealing process. J. Hazard. Mater. 2014, 278, 382−390. (20) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982; DOI: 10.1002/bbpc.19820861019. (21) Xiong, L. B.; Li, J. L.; Yang, B.; Yu, Y. Ti3+ in the surface of titanium dioxide: generation, properties and photocatalytic application. J. Nanomater. 2012, 2012, 831524. (22) Wong, C. P. P.; Lai, C. W.; Lee, K. M.; Hamid, S. B. A. Advanced chemical reduction of reduced graphene oxide and its photocatalytic activity in degrading reactive black 5. Materials 2015, 8, 7118−7128. (23) Erjavec, B.; Kaplan, R.; Pintar, A. Effects of heat and peroxide treatment on photocatalytic activity of titanate nanotubes. Catal. Today 2015, 241, 15−24. (24) Kim, C. H.; Kim, B. H.; Yang, K. S. TiO2 nanoparticles loaded on graphene/carbon composite nanofibers by electrospinning for increased photocatalysis. Carbon 2012, 50, 2472−2481. (25) Pan, X.; Yang, M. Q.; Fu, X.; Zhang, N.; Xu, Y. J. Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications. Nanoscale 2013, 5, 3601−3614. (26) Park, D. R.; Zhang, J.; Ikeue, K.; Yamashita, H.; Anpo, M. Photocatalytic oxidation of ethylene to CO2 and H2O on ultrafine powdered TiO2 photocatalysts in the presence of O2 and H2O. J. Catal. 1999, 185, 114−119. (27) Suriye, K.; Praserthdam, P.; Jongsomjit, B. Control of Ti3+surface defect on TiO2 nanocrystal using various calcination atmospheres as the first step for surface defect creation and its application in photocatalysis. Appl. Surf. Sci. 2007, 253, 3849−3855. (28) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69−96. (29) Luo, L.; Yang, Y.; Xiao, M.; Bian, L.; Yuan, B.; Liu, Y.; Jiang, F.; Pan, X. A novel biotemplated synthesis of TiO2/wood charcoal

catalysts in the water treatment sector and new approaches for organic synthesis. This work makes an important contribution to the application of renewable, biomass-based materials as well as their utilization in effective photocatalytic processes for environmental management.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01251. Detailed experimental procedures for materials characterization and measurements; nitrogen adsorption−desorption isotherms over all tested photocatalysts; table with characteristics of four standard biochars; table with the BET surface area of biochar and TiO2/Biochar materials; phenol adsorption equilibrium curves over all tested photocatalysts in the dark; evaluation of the effect of different scavengers on the photodegradation of phenol in aqueous phase; XRF analysis of the aqueous solution; table with results of extracting phenol from samples’ surface; COD removal; photoluminescence spectra; table with HR XPS (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Juan Carlos Colmenares: 0000-0003-3701-6340 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Centre (NCN) in Poland within research project 2015/17/N/ST5/ 03330. We also thank the COST Association (Action FP1306) for supporting the dissemination of this work. The authors thank Dr. K. Sobczak, Institute of Physics, PAS in Poland for HRTEM images.

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